cifs: Add new mount parameter "acdirmax" to allow caching directory metadata
nfs and cifs on Linux currently have a mount parameter "actimeo" to control metadata (attribute) caching but cifs does not have additional mount parameters to allow distinguishing between caching directory metadata (e.g. needed to revalidate paths) and that for files. Add new mount parameter "acdirmax" to allow caching metadata for directories more loosely than file data. NFS adjusts metadata caching from acdirmin to acdirmax (and another two mount parms for files) but to reduce complexity, it is safer to just introduce the one mount parm to allow caching directories longer. The defaults for acdirmax and actimeo (for cifs.ko) are conservative, 1 second (NFS defaults acdirmax to 60 seconds). For many workloads, setting acdirmax to a higher value is safe and will improve performance. This patch leaves unchanged the default values for caching metadata for files and directories but gives the user more flexibility in adjusting them safely for their workload via the new mount parm. Signed-off-by: NSteve French <stfrench@microsoft.com> Reviewed-by: NRonnie Sahlberg <lsahlber@redhat.com> Reviewed-By: NTom Talpey <tom@talpey.com>
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